Photoacoustic spectroscopy detection is applied for the analysis of various media including solids, liquids, biological tissues, and gases. Photoacoustic gas sensors operate by detecting acoustic vibrations induced by the modulated optical radiation in an analyzed gas sample. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is one of the most sensitive photoacoustic detection techniques using an oscillator like a quartz tuning fork as the sharply resonant acoustic transducer. This method was first presented by A. Kosterev et al. (Opt. Lett. 27, 1902-1904 (2002) and U.S. Pat. No. 7,245,380 B2). QEPAS detection is less sensitive to environmental noise and has the advantages of small size, low cost, and ease of fabrication compared with traditional photoacoustic gas sensors. Numerous QEPAS-based gas sensors have been developed for various gas sensing applications with a normalized noise equivalent absorption (NNEA) coefficient in the range of 10−10-10−7 cm−1 WhiFiz as reviewed in Sensors 14, 6165 (2014) The previous QEPAS sensors using on-beam or off-beam detection schemes (A. Kosterev et al., Opt. Lett. 27, 1902 (2002); K. Liu et al., Opt. Lett. 34, 1594 (2009)) have mostly adopted the open-path optical configuration. Complicated optical alignment, a precise focusing system, and excitation sources with high spatial radiation quality were required to make the laser beam pass through a gap of 300-μm wide between the prongs of the quartz tuning fork and the micro-resonator tubes without touching any surfaces. An additional visualization system is sometimes required for the sensor setup.
Gas absorption by an evanescent field has been demonstrated with a palladium film deposited at a core-exposed fiber (M. Tabib-Azar el al., Sensor. Actuat. B-Chem. 56, 158 (1999)) or the D-shaped optical fiber (G. Stewart et al., Sensor. Actuat. B-Chem. 38; 42 (1997)). The sensitivity of evanescent-wave sensors is determined by the fraction of the optical power in the evanescent field. In the D-fiber evanescent-wave absorption sensor used for methane detection, due to the inherently low evanescent field, an extra sot-gel process was applied to coat the flat surface of the D-fiber to enhance the sensitivity. In the evanescent-wave hydrogen sensor, by depositing palladium over an exposed core region of a multimode fiber, the hydrogen can be detected based on evanescent field interaction with the palladium coating. However, microfibers made by the mechanical processing or chemical etching methods mentioned above suffer from the issue of fragility, Microfihers can also be obtained using the flame-brushing technique as described in Opt. Lett. 35, 85 (2010). The silica fiber taper fabricated using the flame-brushing method has the advantages of a high evanescent field and good mechanical properties.
The tapered microfibers provide an alternative method for photoacoustic detection with evanescent field interactions. A C2H2 photoacoustic sensor using tapered microfibers has been demonstrated by employing a full-taper optical fiber. However, only a bare quartz tuning fork was used in that sensor without micro-resonators. This caused a much lower sensitivity compared to the traditional open-path QEPAS sensors as presented by Y. Cao et al. in Opt Lett 37, 214-216 (2012).